This application claims the priority of Japanese Patent Application No. 2001-350913 filed on Nov. 16, 2001, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a single-focus lens suitable as a taking lens for an imaging device having a small size; and, in particular, to a single-focus lens with a simple configuration having a high optical performance.
2. Description of the Prior Art
Recently, along with the advance in CCD manufacturing techniques, small-sized CCD cameras such as electronic still cameras have begun utilizing a high-density CCD whose number of pixels reaches the order of one million while each pixel reduces its size. Consequently, taking lenses for CCD cameras are desired to have a high resolution and high performances.
Known as taking lenses responding to such a demand are those disclosed in Japanese Unexamined Patent Publication No. HEI 5-157962 and No. HEI 11-125767, for example. The former is a front shutter type taking lens of four-group, five-lens configuration in which a stop is disposed on the object side of the first lens group; whereas the latter is a taking lens of two-group, six-element configuration in which a stop is disposed between the first and second lens groups.
While the taking lenses for CCD cameras are desired to have high optical performances, there are strong demands for making them smaller and less expensive. Demanded are taking lenses having high optical performances on a par with those of conventional ones while further reducing the number of lenses so as to achieve a smaller size and a lower cost.
In view of the circumstances mentioned above, it is an object of the present invention to provide a single-focus lens having a simple three-lens configuration which can achieve a smaller size while favorably correcting aberration, as a taking lens for a small-sized imaging device for a digital camera or the like.
The present invention provides a single-focus lens comprising, successively from the object side, a first lens with a positive refracting power having at least one aspheric surface, a stop, a second lens with a negative refracting power, and a third lens with a positive refracting power having at least one aspheric surface; and satisfying the following conditional expression (1):
0.3 less than fxe2x80x2/fxe2x80x223 less than 0.8xe2x80x83xe2x80x83(1)
where
fxe2x80x2 is the focal length of the lens as a whole; and
fxe2x80x223 is the composite focal length of the second and third lenses.
Preferably, the first lens has a meniscus form with a concave surface on the object side, the second lens has a concave surface on the object side, and the third lens has a convex surface on the image surface side, while the single-focus lens satisfies the following conditional expression (2):
2.0 less than |R11+R12|/|R11xe2x88x92R12| less than 8.0 xe2x80x83xe2x80x83(2)
where
R11 is the radius of curvature of the object-side surface of the first lens; and
R12 is the radius of curvature of the image-surface-side surface of the first lens.
More preferably, the single-focus lens satisfies the following conditional expressions (3) to (6):
1.70 less than Nd1xe2x80x83xe2x80x83(3)
1.60 less than Nd3xe2x80x83xe2x80x83(4)
35 greater than xcexdd1xe2x80x83xe2x80x83(5)
40 less than xcexdd3xe2x80x83xe2x80x83(6)
where
Nd1 is the refractive index of the first lens at d-line;
Nd3 is the refractive index of the third lens at d-line;
xcexdd1 is the Abbe number of the first lens at d-line; and
xcexdd3 is the Abbe number of the third lens at d-line.
The single-focus lens may be mounted in a collapsible camera, the first, second, and third lenses being integrally moved on the optical axis so as to shorten the distance between the third lens and an imaging device when the camera collapses.
At least one of lens surfaces of the first, second, and third lenses may be provided with a coating having a function of a low-pass filter and/or an infrared cut filter, so as to eliminate a glass block having a function of a low-pass filter and/or an infrared cut filter between the third lens and the imaging device.